Batteries are multi-component systems where the theoretical voltage and stoichiometric electron transfer are defined by the electrochemically active anode and cathode materials. While the electrolyte may not be considered in stoichiometric electron transfer calculations, it can be a critical factor determining the deliverable energy content of a battery, depending also on the use conditions. Development of ionic liquid- (IL-) based electrolytes has been the subject of recent reports by other researchers, due in part to opportunities for expanded high voltage operating window and improved safety through reduction of flammable solvent content.
The need for understanding the contributions to deliverable energy content of batteries has taken on an increased significance due to an ever expanding range of battery applications, including grid level systems and devices involving aerospace, transportation, portable electronics, and biomedical applications (Bock, D. C., et al., “Batteries Used to Power Implantable Biomedical Devices,” Electrochimica Acta, 84, 155-164 (2012), which is incorporated by reference in its entirety). Along with energy content and power considerations, the safety of batteries under typical use and abuse conditions is a key consideration for practical implementation. Electrolytes and separators are often overlooked when it comes to battery performance because they do not affect the stoichiometric theoretical electron storage capacity of a battery. However, electrolytes and separators can affect charge transport within a battery which ultimately affects deliverable energy content under a specific application.
In addition, reducing the flammability of the battery electrolyte is a sound strategy for increasing the safety of batteries, especially under abuse conditions. Towards this end, ionic liquids (ILs) are being investigated as possible alternatives to conventional electrolytes (Arbizzani, C., et al., “Thermal Stability and Flammability of Electrolytes for Lithium-ion Batteries,” Journal of Power Sources, 196 (10), 4801-4805 (2011); Komaba, S. et al., “Higher Energy and Safety of Lithium-ion Batteries with Ionic Liquid Electrolyte,” Proceedings of SPIE, 76830F-76830F (2010); and Nakagawa, H., et al., “Application of Nonflammable Electrolyte with Room Temperature Ionic Liquids (RTILs) for Lithium-ion Cells,” Journal of Power Sources, 174 (2), 1021-1026 (2007), each of which is incorporated by reference in its entirety).
While ILs have been known for some time, the study of ILs in battery electrolytes is a more recent field of study (Galiński, M., et al., “Ionic liquids as electrolytes,” Electrochimica Acta, 51 (26), 5567-5580 (2006) and Passerini, S., et al., “Ionic Liquid Based Electrolytes for High Energy Electrochemical Storage Devices,” ECS Transactions, 1 (14), 67-71 (2006), each of which is incorporated by reference in its entirety). For example, battery studies of neat ILs, as well as mixed solutions, have been conducted to probe the effects of ILs on battery electrochemistry (Choi, J.-A., et al., “Mixed Electrolytes of Organic Solvents and Ionic Liquid for Rechargeable Lithium-Ion Batteries,” Bulletin of Korean Chemical Society, 31 (11), 3190-3194 (2010); Diaw, M., et al., “Mixed Ionic Liquid as Electrolyte for Lithium Batteries,” Journal of Power Sources, 146 (1-2), 682-684 (2005); Fu, Y., et al., “Vinyl Ethylene Carbonate as an Additive to Ionic Liquid Electrolyte for Lithium Ion Batteries,” Journal of Applied Electrochemistry, 39 (12), 2597-2603 (2009); Kraytsberg, A., et al., “Higher, Stronger, Better . . . A Review of 5 Volt Cathode Materials for Advanced Lithium-Ion Batteries,” Advanced Energy Materials, 2, 922-939 (2012); Kühnel, R. S., et al., “Mixtures of Ionic Liquid and Organic Carbonate as Electrolyte with Improved Safety and Performance for Rechargeable Lithium Batteries,” Electrochimica Acta, 56 (11), 4092-4099 (2011); Lombardo, L., et al., “Mixtures of Ionic Liquid—Alkylcarbonates as Electrolytes for Safe Lithium-ion Batteries,” Journal of Power Sources, 227 (0), 8-14 (2013); Moosbauer, D., et al., “Effect of Ionic Liquids as Additives on Lithium Electrolytes: Conductivity, Electrochemical Stability, and Aluminum Corrosion,” Journal of Chemical & Engineering Data, 55 (5), 1794-1798 (2010); Sato, T., et al., “Ionic Liquids Containing Carbonate Solvent as Electrolytes for Lithium Ion Cells,” Journal of Power Sources, 138 (1-2), 253-261 (2004); Xu, J., et al., “Additive-containing Ionic Liquid Electrolytes for Secondary Lithium Battery,” Journal of Power Sources, 160 (1), 621-626 (2006); Fox, E. T., et al., “Physicochemical Properties of Binary Ionic Liquid—Aprotic Solvent Electrolyte Mixtures,” The Journal of Physical Chemistry C, 117 (1), 78-84 (2012); and Xiang, H. F., et al., “Improving Electrochemical Properties of Room Temperature Ionic Liquid (RTIL) Based Electrolyte for Li-ion Batteries,” Electrochimica Acta 2010, 55 (18), 5204-5209 (2010), each of which is incorporated by reference in its entirety). ILs are said to be “neat” if no other solvents are added.
Recent reports involving ILs in battery electrolytes include systematic investigations of IL-solvent mixtures, in which the electrochemical stabilities and conductivities of the mixtures were assessed and correlated with physical properties or structural characteristics of the ILs (Di Leo, R. A., et al., “Battery Electrolytes Based on Saturated Ring Ionic Liquids: Physical and Electrochemical Properties,” Electrochimica Acta, 109, 27-32 (2013) (“Di Leo 2013A”) and Di Leo, R. A., et al., “Battery Electrolytes Based on Unsaturated Ring Ionic Liquids: Conductivity and Electrochemical Stability,” Journal of The Electrochemical Society, 160 (9), A1399-A1405 (2013) (“Di Leo 2013B”), each of which is incorporated by reference in its entirety). In that work, the physical and electrochemical properties of a series of ILs based on imidazolium and pyridinium cations with tetrafluoroborate and bis(trifluoromethanesulfonyl) imide anions neat and mixed with ethylene carbonate or propylene carbonate were reported. Higher conductivities were observed with imidazolium cations, tetrafluoroborate (BF4−) anions, and shorter chain-length substituents, while lower conductivities were observed with pyridinium cations, bis(trifluoromethanesulfonyl) imide (TFSI−) anions, and longer chain-length substituents. Investigation of ILs based on saturated ring cations, piperidinium and pyrrolidinium, was also conducted and showed further improvement of electrochemical stability.
Investigating physical and electrochemical properties of ILs and IL-based electrolytes in a systematic way provides the insight necessary to tune ILs for various battery applications. Conductivities of electrolytes are often a primary focus of electrolyte studies, because conductivity directly affects charge transport. However, another electrolyte property which is less studied is the ability of an electrolyte to wet the active and inactive surfaces in a battery. Electrolyte wetting properties, as determined by contact angle can be an illustrative measurement to assess electrolyte-electrode and electrolyte-separator compatibility and ultimately fundamental battery electrochemistry properties. Some previous studies of the ability of ILs to wet surfaces have been reported (Restolho, J., et al., “On the Interfacial Behavior of Ionic Liquids: Surface Tensions and Contact Angles,” Journal of Colloid and Interface Science, 340 (1), 82-86 (2009); Batchelor, T., et al., Wetting Study of Imidazolium Ionic Liquids,” Journal of Colloid and Interface Science, 330 (2), 415-420 (2009); Restolho, J., et al., “Electrowetting of Ionic Liquids: Contact Angle Saturation and Irreversibility,” The Journal of Physical Chemistry C, 113 (21), 9321-9327 (2009); Zhang, F.-C., et al., “Morphology and Wettability of [Bmim][PF6] Ionic Liquid on HOPG Substrate,” Chinese Physical Letters, 27 (8), 086101-1 (2010); Zhang, S., et al., “Enhanced and Reversible Contact Angle Modulation of Ionic Liquids in Oil and Under AC Electric Field,” ChemPhysChem, 11 (11), 2327-2331 (2010); Carrera, G. a. V. S. M., et al., “Interfacial Properties, Densities, and Contact Angles of Task Specific Ionic Liquids,” Journal of Chemical & Engineering Data, 55 (2), 609-615 (2009); and Du, B., “Preparation and Characterisation of Self-assembled Monolayers of Long-chain Alkyl Imidazolium-based Ionic Liquids on Glass Surface,” Journal of Chemical Research, 34 (10), 585-588 (2010), each of which is incorporated by reference in its entirety), however, fewer reports address the contact angle of ILs on substrates relevant to lithium ion batteries (Stefan, C. S., et al., “Are Ionic Liquids Based on Pyrrolidinium Imide Able to Wet Separators and Electrodes Used for Li-ion Batteries?” Journal of Power Sources, 189 (2), 1174-1178 (2009), which is incorporated by reference in its entirety).
Ionic liquids continue to hold interest as possible electrolytes for lithium-based batteries. Therefore, characterization of ILs with regard to their conductivity, electrochemical stability, and thermal safety has been pursued (Di Leo 2013A and Di Leo 2013B). Prior reports indicate that saturated cation-based ILs, in particular pyrrolidinium, exhibit high upper voltage limits of stability as well as wide windows of voltage stability (Di Leo 2013A; Di Leo 2013B; Sakaebe, H., et al., “N-Methyl-N-propylpiperidinium Bis(trifluoromethanesulfonyl)imide (PP13-TFSI)—Novel Electrolyte Base for Li Battery,” Electrochemistry Communications, 5 (7), 594-598 (2003); Monaco, S., et al., “An Electrochemical Study of Oxygen Reduction in Pyrrolidinium-based Ionic Liquids for Lithium/Oxygen Batteries,” Electrochimica Acta, 83 (0), 94-104 (2012); and Zhou, Q., et al., “Physical and Electrochemical Properties of N-Alkyl-N-methylpyrrolidinium Bis(fluorosulfonyl)imide Ionic Liquids: PY13FSI and PY14FSI,” The Journal of Physical Chemistry B, 112 (43), 13577-13580 (2008), each of which is incorporated by reference in its entirety). The imidazolium-based ILs demonstrate high conductivities compared to ILs based on other cations.
Most of the recent research and commercial development of electrochemical storage devices has focused on materials that are primarily suitable for use in portable electronics, for vehicle propulsion, and for back-up grid storage, where the amount of energy storage per unit weight or volume (energy density), cost, and stability are typically critical issues. Such electrochemical storage devices primarily rely on the reversible insertion of lithium ions, and typically comprise a metal-oxide cathode, a carbon-based anode, and an electrolyte containing lithium salt. Although these batteries demonstrate high capacity, limited availability of natural lithium may result in prohibitive costs for large-scale energy storage. Due to its reactivity with water, lithium may also pose potential safety hazards.
Lead-acid electrochemical storage devices, which typically comprise lead plates in an electrolyte containing sulfuric acid, are an alternative to lithium ion batteries. Lead-acid electrochemical storage devices are less expensive to produce than lithium ion batteries, but exhibit low energy density, often corrode, and offer the risk of explosion from “gassing”—the accumulation of hydrogen gas when water inside the device is electrolyzed. Additionally, lead-acid electrochemical storage devices contain toxic products, raising environmental and health concerns.
In an effort to increase cost efficiency, Di Leo et al. (Di Leo 2013A and Di Leo 2013B) explored increasing performance, and, ultimately, increasing the cost efficiency, of lithium ion batteries by using various electrolytes. Specifically, Di Leo et al. (Di Leo 2013A and Di Leo 2013B) discussed using ILs containing piperidinium-based, pyrrolidinium-based, imidazolium-based, or pyridinium-based cations in mixtures with conventional carbonates and lithium salts.
The effect of substituent chain length on conductivity and other properties of ILs has been previously assessed (Montanino, M., et al., “The Role of the Cation Aliphatic Side Chain Length in Piperidinium Bis(trifluoromethansulfonyl)imide Ionic Liquids,” Electrochimica Acta, 57 (0), 153-159 (2011); Fox, E. T., et al., “Tuning Binary Ionic Liquid Mixtures: Linking Alkyl Chain Length to Phase Behavior and Ionic Conductivity,” The Journal of Physical Chemistry C, 116 (8), 5270-5274 (2012); and Tokuda, H., et al., “Physicochemical Properties and Structures of Room Temperature Ionic Liquids. 1. Variation of Anionic Species,” The Journal of Physical Chemistry B, 108 (42), 16593-16600 (2004), each of which is incorporated by reference in its entirety).
Generally, these electrolytes display higher electrochemical stability at higher voltages. This greater stability at higher voltages enables the deployment of high energy-density cathodes in lithium ion batteries, previously considered unsuitable for use with conventional electrolytes. However, there remains a need for an electrochemical storage device that is not as costly to produce as lithium-based batteries.